Cemented carbides have proven value for use in cutting tools due to their extremely high wear-resistance, high impact-resistance and generally high strength. It would be most convenient if the technology of cemented carbides could be transferred directly for use as a wear material in the construction of moving parts of a rotary engine. Unfortunately, this has not been possible because certain environmental conditions and design goals of a rotary engine differ radically from the conditions and goals of a cutting tool.
Although the strength and hardness of a cemented carbide is useable, the cermet must no longer function to cut another contacting surface. For example, although a good cemented carbide will have high strength enabling it to be used for a dynamic apex seal of a rotary engine, it is important that the seal element have a compatible frictional wear characteristic with respect to the opposite bearing surface so as to promote a gas-tight seal. Thus, while hardness is important, it is equally important that there be a certain amount of inherent lubricity in the composition of the material to facilitate long life under constant rubbing conditions.
Thermal conditions in a combustion zone of a rotary engine reach severe levels which can cause heat checking in equivalent hard materials. Known cermet materials normally are expected to suffer cracking under such thermal conditions. In addition, the rotor of a rotary internal combustion engine is typically eccentrically mounted so that the apices of the rotor may traverse contours of an epitrochoid formed on the inner wall of the rotor housing. The dynamic forces imposed upon the apex seals, which are adapted to slightly shift within grooves of the rotor, cause the seal element to make unwanted chatter marks on the epitrochid surface of the rotor housing. Eventually, the depth of these chatter marks increase so that sealing effectiveness is dissipated and the engine loses considerable efficiency. It is thought that two aspects play an important role in the problem of chatter, namely inertial or dynamic mass weight of the seal element and the relative freedom from high interengaging friction.
Graphite, including other well ordered crystallites have been known for their lubricating characteristic or freedom from high interengaging friction. The atomic structure is such that slip planes are easily set up parallel to the rubbed surface. Consideration as to the presence and effect of excess carbon (graphite) on the properties of cemented carbides, particularly sintered tungsten carbide, has been well documented to reveal the state of the art. In all cases, the art holds the view that free carbon in cemented carbides is generally detrimental causing a drop in strength, hardness, and impact resistance. For example, the mechanical properties of sintered tungsten carbide-cobalt alloys have been analyzed in an article by Al D. Brownlee, R. Edwards and T. Raine, Symposium on Powder Metallurgy, page 302-304, 1954. It is stated, starting on page 303, that "the presence of free carbon tends to encourage the grain growth of tungsten carbide. This leads to a fall in hardness but the carbon itself, if present only in small amounts, does not noticeably affect the hardness. However, if the excess is so great that it causes the formation of `rosettes` the hardness will be very greatly reduced. The presence of excess carbon has two conflicting effects on the transverse rupture strength of these alloys. The increase in grain size of the tungsten carbide tends to increase the transverse rupture strength, but at the same time the precipitation of graphite in the form of clusters and rosettes forms weak points in the material, which leads to a lowering of the transverse rupture strength. The net effect is that, as the carbon content increases, the transverse rupture strength first increases very slightly and then falls off rapidly."
Again in an article by D. N. French and D. A. Thomas, International Journal of Powder Metallurgy (III), 1967, it is concluded on page 14 that "Excess carbon-type defects reduce the transverse rupture strength and impact strength of tungsten carbide -- 10 wt. % cobalt alloys". Lastly in an article in the Journal of Metals, 1954, by J. Gurland, Transactions of AIME, page 200, and particularly on page 287, it is stated "graphite moderately decreases strength and hardness". This also is in reference to a tungsten carbide -- cobalt alloy. Thus, the prior art has not appreciated the virtue of excess free carbon for rotary engine applications.
A typical commerical seal of cemented carbide comprises 35% titanium carbide, 5.75% chromium, 2% molybdenum, 0.56% carbon and the balance iron; this cermet is commonly referred to as Ferrotic CM.